1. Field of the Invention
Enzymatic reactions are increasingly used for processing materials on an industrial scale. An important commercial step was the development of immobilized enzymes, i.e., enzymes that are physically attached to a solid support—either by adsorption or chemical bonds—to facilitate the separation of the enzyme from the reaction solution Immobilization also enables multiple or repetitive use of the enzymes and often increases the stability of the enzymes.
2. Description of Related Art
Several types of reactors are used in conjunction with immobilized enzymes: packed bed reactors (also referred to as fixed bed reactors), fluidized bed reactors, stirred batch reactors, continuously stirred tank reactors, and membrane reactors. In large-scale production, packed bed reactors are commonly used for particular applications. In such reactors, the immobilized enzyme is packed in a column or flat bed while substrate and product streams are pumped into and out of the reactor, respectively. The main advantages of this type of reactor are an easy adaptation to larger scales, high efficiency, low costs, ease of operation and also an enhanced surface area per unit volume compared to membrane reactor systems (cf. W. M. Willis and A. G. Marangoni, Enzymatic Interesterification, in: Food Lipids—Chemistry, Nutrition and Biotechnology, edited by C. C. Akok and D. B. Min, pages 839-875).
A type of reaction in which the use of enzymes as catalysts has become increasingly important in recent years in the oils and fats industry is interesterification. Interesterification is the exchange of acyl groups between an ester and an acid (acidolysis), an ester and an alcohol (alcoholysis) or between two esters (transesterification). The enzymes capable of interesterification reactions are lipases such as glycerol ester hydrolases (EC 3.1.1.3). These enzymes are predominantly obtained from bacterial, yeast, and fungal sources. These microorganisms secrete lipases into their environment to digest lipid materials for subsequent uptake. In an aqueous environment, lipases catalyse the hydrolysis of triacylglycerides to produce diacylglycerides, monoacylglycerides, glycerol and free fatty acids. However, under water-limiting conditions, the reverse reaction, the synthesis of esters, can also be achieved. Therefore, the direction of the reaction can be manipulated by regulating the water activity. At very low water concentrations ester synthesis predominates, above a water content of more than a few percent hydrolysis is the prevailing reaction, and in between, usually at a water content of less than 1% (w/v), transesterification is most effective.
Owusu-Ansar described in 1994 the use of immobilized lipases in a stirred tank reactor for the large-scale production of cocoa butter equivalents by lipase-catalysed interesterification (in: B. S. Carmail and Y. Kakuda (eds.) Technological Advances in Improved and Alternative Sources of Lipids, pages 360-389).
A packed bed interesterification process has been disclosed in WO 83/03844 wherein triacylglycerides are dissolved in a polar organic solvent. The lipase was immobilized by precipitation onto kieselguhr, hydroxyapatite or alumina particles.
WO 97/01632 is directed to another process for immobilization of an enzyme, specifically, a lipase or a phospholipase, for the processing of triglyceride oils.
Unilever further disclosed a two-stage process for the production of cocoa butter equivalents and Betapol using packed bed columns (P. Quinlan & S. Moore, Inform 4, 580-585 (1993); A. Rozendaal & A. R. Macrae, in: F D. Gansdone & F B. Padlee (eds.) Lipid Technologies and Applications, 1997, pages 223-263).
X. Xu describes further processes with immobilized lipases, among them a packed bed reactor with a capacity of 10 kg/day (in U. T. Bomscheuer, Enzymes in Lipid Modification, 2000, pages 190-215).
The existing packed bed reactors suffer from the problem of a decrease in conversion rate over time. Though some enzymes such as lipases are quite stable, their efficacy inevitably drops over time, which results in lower reaction rates and non-uniform product quality. This lowering of quality can be counteracted by decreasing the flow rate and thereby increasing the residence time of the reactants in the reactor. Such a reduction in the flow rate, however, leads to a non-uniform output rate and while it can reduce the fluctuations in product quality, it cannot avoid them completely. Furthermore, such a process often uses a number of discrete reactors in series which requires a complex flow control in order to run and maintain the system. The current invention solves these and related problems by using a new type of packed bed column process for enzymatic modification of a substrate.